Awesome Analog Synthesizer/Organ Using Only Discrete Components

Analog synthesizers are very cool, but also quite difficult to make.

So I wanted to make one as simple as it can get, so its functioning can be easily understandable.

For it to work, you need a few basic sub-circuits: A simple oscillator with resistor selectable oscillating frequency, some keys, and a basic amplifier circuit.

If you use some conductive pads instead of push buttons for the keys, you could make your version of the very cool

Stylophone!

In this instructable we will learn how to make it and we will learn how it works.

The instructable is meant for beginner to intermediate electronics enthusiasts.

Step 1: Tools Needed

You will need a soldering iron and some prototyping boards, or you can assemble it on the breadboard.

If you are a little more advanced, I will provide files for etching your own PCB.

Step 2: Starting With an Oscillator

The heart of the synthesizer is an Astable Multivibrator circuit made with an operational amplifier. On the internet you will find very long and detailed derivations of its operation, but I will try to explain its working in a more simple way.

The oscillator consists of a few resistors and one capacitor.

The op-amp comparator circuit is configured as a Schmitt trigger that uses positive feedback provided by resistors R1 and R2 to generate hysteresis. This resistive network is connected between the amplifiers output and non-inverting (+) input. When Vo (output voltage) is saturated at the positive supply rail, a positive voltage is applied to the op-amps non-inverting input. Likewise, when Vo is saturated to the negative supply rail, a negative voltage is applied to the op-amps non-inverting input.

This voltage slowly charges and discharges the capacitor at the (-) input through the Rf resistor. Lets say we start with op-amps output at positive saturation voltage (+Vsat). The capacitor is being charged and its voltage (Vc) is slowly rising. In the mean time R1 and R2 form a voltage divider with its voltage output (Vdiv)at a stable value somewhere between output saturation voltage (+Vsat) and 0V. When the capacitor voltage exceeds the voltage of the R1 and R2 voltage divider, the op-amp inverts its state to negative saturation voltage (-Vsat). Then the capacitor is being discharged through the Rf resistor until its voltage (Vc) is lower than the R1 and R2 dividers voltage (Vdiv). Then it again flips its state to the initial state (+Vsat). And so on and on.

This in fact produces square-wave voltage output voltage of the oscillator and if it is of the right frequency, it produces an audible tone.

Step 3: Calculating the Frequencies

The oscillator frequency can be calculated via the equation in the picture above.

You can tune this synth whatever you like.

I wanted to tune it in C major scale - all the white keys on the piano. This way, there are no "wrong" tones and it is easy to play for kids.

So I searched online for the list of frequencies for the specific tones and I decided to tune the thing from C4 to C5 note.

I made the calculations for the needed resistor. I did it fancy and calculated it with Matlab (Octave).

For the R1 and R2 resistor divider I chose 22k ohm resistors, for the capacitor I chose 100nF cap.

Here is the code if you are too lazy to do it by hand with a calculator.
Or you can just use the flipped equation for the manual resistor calculation.

R1=220e3;
R2=220e3;

lambda=R1/(R1+R2);

C=100e-9;

f=[261.63 293.66 329.63 349.23 392 440 493.88 523.25]; %list of frequencies

R=1./(f.*2.*C.*log((1+lambda)/(1-lambda)))

Here are the results:

C4 = 17395 ohm

D4 = 15498 ohm

E4 =13806 ohm

F4 = 13032 ohm

G4 = 11610 ohm

A4 = 10343 ohm

B4 = 9215 ohm

C5 = 8697 ohm

Of course I needed to round the values to the nearest resistor values. I used standard E12 resistor series which is the most often found in hobby parts box. Because E12 resistor series is pretty coarse, I used 2 resistors in series for each value in order to get closer to the desired resistance and the synth will be more in tune this way.

C4 = 2.2k + 15k ohm
D4 = 15k + 470 ohm

E4 =8.2k + 5.6k ohm

F4 = 12k + 1k ohm

G4 = 4.7k + 6.8k ohm

A4 = 10k + 330 ohm

B4 = 8.2k + 1k ohm

C5 = 8.2k + 470 ohm

Step 4: The Finished Oscillator Schematic

Here is the schematic for oscillator part.

With the individual keys, you select the desired resistance and the desired tone is produced.

This schematic explains why you get high pitched sounds when pressing multiple keys at once. By pressing multiple keys at once, you connect more branches of the resistors in parallel and effectively connecting them in parallel, reducing the total resistance. Lower resistance produces higher pitched tone.

Step 5: The Speaker Amplifier

The speaker amplifier could be done even simpler, but I decided to make a true AB class amplifier stage.

The stage consists of PNP and NPN transistors, coupling capacitors and two bias resistors and diodes.

Very basic but it works well.

In front of the amplifier stage I put a 100k logarithmic (audio) potentiometer for adjusting the volume.

Because the potentiometer on its own in the circuit would de-tune the oscillator(added resistance), I slapped an op-amp buffer in front of it which introduces high input resistance for the circuit in front of it and low impedance for the circuits after it.

Basically a buffer is an amplifier with a gain of 1.

The opamp I am using is TL072 which has two amplifier circuits in it, so this is all we need.

Step 6: Auxilliary Stuff

On the left side of the image there are the input connector headers, where you connect the power supply.

They are followed by two diodes which protect the circuit for accidental connection of the wrong polarity power supply.

I also added two LEDs for indicating the presence of each power line.

Step 7: Full Schematic

Here is the finished schematic.

Step 8: The Power Supply

The circuit requires symmetrical power supply.

You need +12V and -12V (9V would also work).

I used some old power supply from a broken inkjet printer, since it had +12V and -12V rails (see the photos)

But you can also make a symmetrical +-12V power supply from a single 24V one using the schematic above.

But just don't forget to mount a heatsink to the 7812 regulator.

Or you can connect in series two isolated 12V power supplies.

Step 9: The PCB

If you like to etch your own PCBs you can find the file for printing here. I used 10x10mm pushbuttons for the keys.

Many people wanted to know where to find buttons with a nice big cap.
Here I managed to find similar pushbuttons you can use for the keyboard:

https://www.banggood.com/custlink/GvDmqJEpth

They should also fit on a breadboard!

This is affiliate link - you pay the same price as without the link, but I get a small commission so I can buy more components for projects to come :)

For the capacitor selector, I soldered the header so I can quickly change the capacitors.

On the other side, the circuit is simple enough so that you can assemble it on the breadboard or a prototyping solder board. It would be even easier to tinker with and swapping the components for different effects.

For the speaker I recycled an old internal PC speaker, I made a simple 3D printed enclosure for it.